P

example is shown in Fig. 23.24. Since the magnetic mirrors are near the magnetic poles, the aurorae are most prominent close to the north and south poles. In addition, aurorae are strongest when there are a lot of charged particles arriving from the Sun.

23.5 I Tides

A number of phenomena on Earth depend on the fact that the gravitational force exerted by the Moon (or the Sun) on the Earth is slightly different at different parts of the Earth. As we have already discussed (Chapter 12), any effect which depends on the difference between the gravitational forces on opposite sides on an object is called a tidal effect.

If we have an object of mass m, a distance r from an object of mass M, the force on the object of mass m is

GMm dF dr

2GMm

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